In order to meet a variety of forums and new technologies related to the 4th generation mobile communications, the 3rd Generation Partnership Project (3GPP) which aims to provide technical specifications of the 3rd generation mobile communications system has proceeded with research for the Long Term Evolution/System Architecture Evolution (LTE/SAE) technologies since year-end 2004 as a part of efforts to optimize and enhance performances of the 3GPP technologies.
The SAE mainly led by the 3GPP SA WG2 relates to research of network technologies which aims to determine a network structure together with the LTE work of the 3GPP TSG RAN and to support mobility between networks of different versions. Recently, the SAE has been considered one of the essential standardization issues of the 3GPP. Such work is to develop the 3GPP to be a system based on the IP and supporting a variety of radio (wireless) connection technologies, and has progressed with the aim of an optimized packet-based system capable of minimizing a transmission delay with enhanced data transmission capability.
The SAE upper level reference model defined by the 3GPP SA WG2 may include a non-roaming case and roaming cases with a variety of scenarios. Detailed descriptions thereof are given in 3GPP TS 23.400a and TS 23.400b. FIG. 1 is a schematic reconfiguration diagram of such network structure.
FIG. 1 is view of an evolved mobile communication network.
One of the distinctive characteristics of the network structure shown in FIG. 1 is that it is based on a 2 tier model having an eNode B of the Evolved UTRAN and a gateway of the core network. The eNode B 20 has a similar function, although not exactly the same, to the eNode B and RNC of the existing UMTS system, and the gateway has a function similar to the SGSN/GGSN of the existing system.
Another distinctive characteristic is that different interfaces are exchanged by the control plane and the user plane between the access system and the core network. While an lu interface exists between the RNC and SGSN in the existing UMTS system, two separate interfaces, i.e., S1-MME and S1-U, are used in the Evolved Packet Core (SAE) system since the Mobility Management Entity (MME) 51 which handles the processing of a control signal is structured to be separated from the gateway (GW).
For the GW, there are two types of gateways: a Serving Gateway (hereinafter, ‘S-GW’) 52 and a Packet Data Network gateway (hereinafter, ‘PDN-GW’ or ‘P-GW’) 53.
FIG. 2 shows the structure and communication process of an MTC device.
A Machine Type Communication (MTC) device may be used in a mobile communication system. The MTC refers to data communications between machines performed without human interference, and a device used for these communications is referred to as an MTC device. A service provided by the MTC device is different from a communication service performed with human interference, and may be applied to a variety of services.
The aforementioned MTC device is a communication device is a communication device that performs communication between machines, which is not much different from a UE that needs human interaction, except that it needs no human interaction. That is, the MTC device may correspond to a UE that needs no human interaction. However, from the viewpoint that no human interaction is needed, if a message transmission/reception method (e.g., paging message transmission/reception method) for a UE that needs human interaction is fully applied to the MTC device, some problems may occur.
Referring to FIG. 2, when a measuring service, a road information service, a user electronic equipment calibration service, or the like, provided by the MTC device, received by an eNB, the eNB may transmit it to an MTC server, and therefore the MTC user may use the service.
It is often the case that the MTC device performs communication alone in a place that needs no human interference since it performs communication without human interaction.
FIG. 3 shows an exemplary structure of a radio interface protocol in a control plane between the UE and the base station, and FIG. 4 shows an exemplary structure of a radio interface protocol in a user plane between the UE and the base station.
The radio interface protocols are based on the 3GPP radio access network standards. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting data information and a control plane (C-plane) for transmitting control signals (signaling).
The protocol layers can be categorized as a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the communication system.
The layers of the radio protocol control plane of FIG. 3 and those of the radio protocol user plane will be described as follows.
The physical layer, the first layer, provides an information transfer service by using a physical channel. The physical layer and an upper layer called a medium access control (MAC) layer are connected via a transport channel. Data is transferred between the MAC layer and the physical layer via the transport channel. Between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transferred via the physical channel.
The physical channel is composed of a number of subframes present in a time axis and a number of subcarriers present in a frequency axis. Here, a single subframe includes a plurality of symbols and a plurality of subcarriers in the time axis. A single subframe includes a plurality of resource blocks, and a single resource bock includes a plurality of symbols and a plurality of subcarriers. A single resource block is called a slot and has a length of 0.5 ms temporally. A TTI (Transmission Time Interval), a unit time during which data is transmitted, is 1 ms which corresponds to a single subframe.
Physical channels existing in the physical layers of a transmitter and a receiver include an SCH (Synchronization Channel), a PCCPCH (Primary Common Control Physical Channel), an SCCPCH (Secondary Common Control Physical Channel), a DPCH (Dedicated Physical Channel), a PICH (Paging Indicator Channel), a PRACH (Physical Random Access Channel), a PDCCH (Physical Downlink Control Channel), and a PDSCH (Physical Downlink Shared Channel).
The MAC layer, the second layer, is connected with the physical layer through a transport layer, and connected to an upper layer called a radio link control (RLC) layer via a logical channel.
A downlink transport channel for transmitting data from the network to the UE includes a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting a paging message, and a downlink shared channel (DL-SCH) for transmitting user traffic or a control message. The downlink multicast, traffic of a broadcast service, or a control message may be transmitted via the downlink SCH or a separate downlink MCH (Multicast Channel). An uplink transport channel for transmitting data from the UE to the network includes a random access channel (RACH) for transmitting an initial control message and an uplink SCH for transmitting other user traffic or a control message.
The logical channel is divided into a control channel that transmits information of the control plane and a traffic channel that transmits information of the user plane according to a type of transmitted information.
The logical channels, which are at an upper position than the transport channel and mapped to the transport channel, include a BCCH (Broadcast Channel), a PCCH (Paging Control Channel), a CCCH (Common Control Channel), an MCCH (Multicast Control Channel), an MTCH (Multicast Traffic Channel), a DCCH (Dedicated Control Channel), and the like.
An RLC (Radio Resource Control) layer, the second layer, supports reliable data transmission, guarantees quality of service (QoS) of each radio bearer (RB), and is responsible for (or handles) data transmission. In order to guarantee RB-specific QoS, the RLC has one or two independent RLC entities for each RB, and in order to support various types of QoS, the RLC layer provides three RLC modes: a TM (Transparent Mode); a UM (Unacknowledged Mode); and an AM (Acknowledged Mode).
A packet data convergence protocol (PDCP) layer, the second layer, performs a function called header compression that reduces the size of a header of an IP packet, which is relatively large and includes unnecessary control information, in order to effectively transmit the IP packet such as an IPv4 or IPv6 in a radio interface having a smaller bandwidth. Also, the PDCP layer is used to cipher data of the C-plane, e.g., an RRC message. The PCP layer also ciphers data of the U-plane.
A radio resource control (RRC) layer located at the uppermost portion of the third layer is defined only in the control plane, and controls a logical channel, a transport channel, and a physical channel in relation to configuration, reconfiguration, and the release or cancellation of radio bearers (RBs). In this case, the RBs refer to a service provided by the second layers of the radio protocol for data transmission between the UE and the E-UTRAN.
When there is an RRC connection between the RRC of the UE and the RRC layer of the wireless network, the UE is in an RRC-connected mode, or otherwise, the UE is in an idle mode.
A non-access stratum (NAS) layer positioned at an upper portion of the RRC layer performs functions such as session management, mobility management, and the like.
The NAS layer illustrated in FIG. 3 will be described in detail.
An eSM (evolved session management) that belongs to the NAS layer performs a function such as a default bearer management, a dedicated bearer management, or the like, and is responsible for (or handles) controlling to allow the UE to use a PS service in a network. Default bearer resource has characteristics in that it is allocated from a network when the UE first accesses a particular packet data network (PDN). In this case, the network allocates an IP address that may be used by the UE to allow the UE to use a data service, and also allocates QoS of a default bearer. In LTE, two types of bearers, i.e., a bearer having guaranteed bit rate (GBR) QoS characteristics that guarantee a particular band width for a data transmission and reception and a non-GBR bearer having best effort QoS characteristics without guaranteeing a bandwidth, are supported. In the case of the default bearer, the non-GBR bearer is allocated. In the case of a dedicated bearer, a bearer having the QoS characteristics of the non-GBR is allocated.
The bearer allocated to the UE by the network is called an evolved packet service (EPS) bearer, and when the network allocates the EPS bearer, the network allocates an ID. This is called an EPS bearer ID. A single EPS bearer has QoS characteristics of a maximum bit rate (MBR) or/and guaranteed bit rate (GBR).
FIG. 5 is a conceptual diagram showing a 3GPP service model for supporting MTC.
Although GSM/UMTS/EPS with the 3GPP standards for supporting MTC are defined to perform communication over a PS network, the present specification describes a method applicable to a CS network as well.
In the current technical specification, the use of an existing 3GPP bearer is suggested for the definition of the network structure. A method using a short message service (SMS) for data exchange between an MTC device and an MTC server was proposed as one of alternative solutions. The use of SMS was proposed, considering that a small amount of digital data including meter reading information and product information will be an object of an MTC application in view of the characteristics of the MTC application, by which an existing SMS method and an IMS-based SMS method can be supported.
In the current 3GPP standards, three architecture models for MTC are defined as follows: a Direct Model, an Indirect Model, a Hybrid Model, and so on. The Direct Model is a model in which an MTC application is connected directly to an UE over a 3GPP network and performs communication under the control of a 3GPP network provider. The Indirect Model includes two models: a model in which an MTC application is connected to an MTC server outside a 3GPP network to perform communication with a UE under the control of an MTC service provider; and a model in which an MTC server exists within a 3GPP network and an MTC application is connected to an UE to perform communication under the control of a 3GPP network provider. The Hybrid Model involves the co-existence of the Direct Model and Indirect Model. For example, user plane is a method of communication using the Direct Model and control plane is a method of communication using the Indirect Model.
As described above, Machine Type Communication (MTC) involves communication performed between machines, which may result in overload in some cases. For example, overload may be generated due to the following reasons:
there is a malfunctioning in the MTC server or MTC application;
an external event triggers MTC devices to attach/connect; and
a large number of MTC devices are configured such that a specific program is repeatedly operated at a specific time.
FIG. 6 shows a network overload state.
As illustrated in FIG. 6, if traffic is overloaded or congested at an interface between the (e)NodeB 20 and the S-GW 52, then downlink data to the MTC device 10 or upload data from the MTC device 10 is failed to be properly transmitted.
Also, if an interface between the S-GW 52 and the PDN-GW 53 or an interface between the PDN-GW 53 and an Internet Protocol (IP) service network of the mobile communication operator is overloaded or congested, then downlink data to the MTC device 10 or upload data from the MTC device 10 is failed to be properly transmitted.
Also, when the MTC device is handed over from a cell being currently serviced to another cell, if the another cell is overloaded, then it will cause a problem of dropping the service of the MTC device.
In order to solve the foregoing problem, mobile communication operators have updated the S-GW 52 and the PDN-GW 53 having high-capacity, but it has a disadvantage of requiring very high cost. Furthermore, it has a disadvantage that the amount of transmitted or received data increases exponentially over time, and then overloaded in a short time.